Handover management in communication systems using unlicensed frequency bands

ABSTRACT

Handovers in a New Radio-Unlicensed Standalone (NR-U SA) communication system are managed by invoking one of a plurality of handover procedures based on conditions related to the communication device and channel status. One example of one of the plurality of handover procedures includes sending a handover command with an inactivity indicator to the UE device where the UE device transitions to an Inactive State (such as RRC_INACTIVE) in response to a handover failure (HOF) resulting from an occupied unlicensed channel. The UE device provides the 1-RNTI allocated by the last serving gNB when resuming from the RRC_INACTIVE to the active state; thereby reducing the latency associated with a new connection from the idle mode. The target gNB uses the 1-RNTI to retrieve the UE context from the last serving gNB.

CLAIM OF PRIORITY

The present application claims the benefit of priority to ProvisionalApplication No. 62/716,713, entitled “Method For Mobility Control In AStandalone New Radio—Unlicensed Network”, filed Aug. 9, 2018, assignedto the assignee hereof and hereby expressly incorporated by reference inits entirety.

FIELD

This invention generally relates to wireless communications and moreparticularly to handover management in communication systems usingunlicensed frequency bands.

BACKGROUND

Wireless technologies, such as WiFi and Bluetooth, utilize unlicensedfrequency bands for communication. In the United States, the unlicensedbands are located in at least the 900 MHz, 2.4 GHz and 5.8 GHz frequencybands. Some communication technologies that have traditionally operatedwithin licensed frequency bands are beginning to use unlicensed bandsfor some communication. For example, the 3rd Generation PartnershipProject (3GPP) has developed Licensed-Assisted Aggregation (LAA) to makeuse of the free unlicensed bands in conjunction with the licensed band.LAA is very similar to Carrier Aggregation (CA) whereby user equipment(UE) devices can simultaneously access multiple cells at the same timeusing a primary cell (PCell) and a secondary cell (SCell). The SCellwhich is typically a small cell, handles much of the broadband load andthe PCell which is typically a macro-cell, handles the controlsignaling. The UE devices do not experience unnecessary increase inhandover failures when moving from one small cell to another. As long asthe UE device remains connected to the PCell (typically a macro-cell),the UE devices do not declare handover failure even if the connection tothe SCell is lost. LAA is designed for fair coexistence among LAAnetworks deployed by different operators and other non-3GPPtechnologies. AS a result, 3GPP requires a listen-before-talk (LBT)procedure which is defined as a mechanism by which an equipment appliesa clear channel assessment (CCA) check or LBT check before using theunlicensed channel.

A newly developing technology studied by the 3GPP and referred to as NewRadio-Unlicensed Standalone (NR-U SA) proposes an NR-based celloperating standalone in unlicensed spectrum and connected to a 5G corenetwork (5G-CN) with priority on frequency bands above 6 GHz. With thissystem, there is no longer an anchor cell (PCell) that is transmittingin a licensed band. As a result, control signaling for mobility isperformed over unlicensed bands.

SUMMARY

Handovers in a New Radio-Unlicensed Standalone (NR-U SA) communicationsystem are managed by invoking one of a plurality of handover proceduresbased on conditions related to the communication device and channelstatus. In one example, one of the plurality of handover proceduresincludes a procedure that involves sending a handover command with aninactivity indicator to the UE device where the UE device transitions toan Inactive State (such as RRC_INACTIVE) in response to a handoverfailure (HOF) resulting from an occupied unlicensed channel. By sendingthe UE to the Inactive state upon handover failure, the UE device willavert from declaring radio link failure (RLF) if the UE device cannotfind an unoccupied target cell prior to T311 timer expiry. While the UEdevice is in the Inactive state, the UE's context is kept at both thesource gNB and the UE device which enables quick reconnection once anunlicensed channel becomes available. Where the UE device declares RLFand transitions to IDLE, then the UE context will be lost and servicewill need to be established as a new connection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example of a communication systemsupporting operation in accordance with 3GPP New Radio-UnlicensedStandalone (NR-U SA) with dynamic mobility management.

FIG. 2 is a block diagram of an example of a base station suitable foruse as each of the base stations.

FIG. 3 is a block diagram of an example of a UE communication devicesuitable for use as each of the communication devices of FIG. 1 .

FIG. 4 is a timing diagram of communication for an example where aconditional handover is authorized by the first base station in acommunication system utilizing unlicensed frequency bands fortransmission.

FIG. 5 is a flow chart of an example of a method of managing handoverswith conditional handover commands.

FIG. 6 is a timing diagram of communication for an example where ahandover command is provided with an inactivity indicator by the firstbase station in a communication system utilizing unlicensed frequencybands for transmission.

FIG. 7 is a flow chart of an example of a method of managing handoverswith handover commands having inactivity indicators.

FIG. 8 is a flow chart of an example of a method of managing measurementreport transmission.

DETAILED DESCRIPTION

As discussed above, in a NR-U SA system, control signaling for mobilityis performed over unlicensed bands. Since the unlicensed band may beoccupied and the UE device must perform LBT procedures prior to anytransmission, the UE device may not be able to send the required controlsignaling in a timely manner. Therefore, one of the distinct differencesbetween LAA and NR-U SA is the lack of a reliable channel for deliveryof control signaling critical to reliable operation. Due to the need forLBT operation for every signaling transmission, the reliability andlatency of the control signaling may be significantly impacted due tomobility operation in the unlicensed band. Currently, the NR mobilityprocedure is designed to combat radio link problems and/or interferencefrom neighboring cells but is not equipped to handle the lack of areliable channel due to channel occupancy by other systems and devices.In particular, handovers are challenging since the existing NR basedmobility is largely based on LTE with the assumption that licensedspectrum will be used for control signaling. It is difficult todetermine whether a signaling failure is due to radio link issues orwhether the channel is just temporary occupied. This issue is especiallycritical under mobility since it is expected that handovers arecompleted prior to T310 expiry once the UE device receives N310consecutive out-of-sync indications from the lower layer. With currentsystems, upon T310 expiry, the UE device will transition to the IDLEstate regardless of whether the cause of the physical layer problem wasdue to radio link issue or an occupied channel.

For the examples discussed herein, handovers are managed by invoking oneof a plurality of handover procedures based on conditions related to thecommunication device and channel status. Examples of conditions that maybe evaluated to select a particular handover procedure may be based onwhether the unlicensed channel is occupied, the UE device's powerconsumption status, mobility status (whether the UE device is stationaryor moving), whether the UE device requires to operate in a licensed banddue to a requirement for critical communication service, andcombinations thereof.

As discussed below, one example of the plurality of handover proceduresincludes providing a conditional handover command associated with ahandover time period established by the serving gNB. After determiningan unlicensed channel is unoccupied, the UE device transmits ameasurement report over the unlicensed channel to the serving gNB. Theserving gNB transmits the conditional handover command associated withthe handover time period.

Another example of the plurality of handover procedures includes sendinga handover command with an inactivity indicator to the UE device wherethe UE device transitions to an Inactive State (such as RRC_INACTIVE) inresponse to a handover failure (HOF) resulting from an occupiedunlicensed channel. By sending the UE to the Inactive state uponhandover failure, the UE device will avert from declaring radio linkfailure (RLF) if the UE device cannot find an unoccupied target cellprior to T311 timer expiry. While the UE device is in the Inactivestate, the UE's context is kept at both the source gNB and the UE devicewhich enables quick reconnection once an unlicensed channel becomesavailable. Where the UE device declares RLF and transitions to IDLE, theUE context will be lost and service will need to be established as a newconnection. Once the UE device is able to camp on a cell with unoccupiedunlicensed channel, the UE device may proceed with the connectionre-establishment procedure to transition from RRC_INACTIVE toRRC_CONNECTED where the UE device provides the I-RNTI allocated by thelast serving gNB when resuming from the RRC_INACTIVE. The target gNBuses the I-RNTI to retrieve the UE context from the last serving gNB. Aslong as the UE device selects a target cell that is within the RNA, thetarget gNB should be able to retrieve the information. Therefore, unlikethe timer for re-establishment, the UE device may resume the connectionwhen the previously occupied channel becomes unoccupied.

FIG. 1 is a block diagram of an example of a communication system 100supporting operation in accordance with 3GPP New Radio-UnlicensedStandalone (NR-U SA) with dynamic mobility management. For NewRadio-Unlicensed standalone deployments, mechanisms for inter-cellhandover between NR-U and NR-U and Inter-RAT handover between NR-U andLTE are needed. Extensions of mobility-related measurement reporting forunlicensed operation including channel occupancy indication and RSSImeasurements will also be required. The communication system 100,therefore, may operate in accordance with one or more revisions of the3GPP NR-U SA communication specification and includes mechanisms formanaging handover failures due to occupied unlicensed channels. Thecommunication system 100 includes a plurality of base stations 102-104providing wireless service to UE devices 106-109 within coverage areas110-112. The UE devices 106-108 communicate with the base stations inone or more unlicensed frequency bands. In some case, one or moreunlicensed frequency channels belonging to the same or different bandsmay be used at each of the based stations 102-104. Furthermore, theunlicensed frequency channels may be provided to each UE devicesimultaneously using carrier aggregation. Therefore, the UE devices106-108 transmit uplink signals in an unlicensed frequency band andreceive downlink signals in an unlicensed frequency band. Typically, thesame unlicensed frequency band is used for uplink and downlink althoughdifferent bands can be used in some situations. An unlicensed frequencyband is any frequency band that does not require the operator to filedirectly with a governmental agency regulating frequency spectrum, suchas the Federal Communications Commission (FCC), in order to use theband. Examples of unlicensed frequency bands U.S. include the 900 MHz,2.4 GHz and 5.8 GHz. Currently developing systems contemplate using atleast the 6 GHz band which in the U.S. ranges between 5925 MHz and 7125MHz and between 5925 MHz and 6425 MHz, in Europe. Portions of bands canalso be used. The range 5150-5925 MHz, or parts thereof, is potentiallyavailable for license-assisted access to unlicensed operation bands.This frequency range can be operated under a license-exempt regime orISM but must be shared with existing mobile services and other incumbentservices. The quality of service offered by a licensed regime,therefore, cannot be matched. Hence, unlicensed access is viewed ascomplementary, and does not reduce the need for additional allocationsfor licensed operation in view of the increased demand for wirelessbroadband access. Other unlicensed frequency bands and portions offrequency bands can also be used. For the examples, herein, operation inthe unlicensed frequency bands requires listen-before-talk (LBT) where atransmitting device must observe a channel to determine that is not inuse before transmitting in that channel.

The base stations 102-104 are any transceivers that provide wirelessservice in a coverage area and can perform the functions describedherein. The base stations 102-104 may be referred to as access points,access nodes, transceiver nodes, eNodeB, eNB, gNB and other termsdepending on the particular system and application. For the example, thebase stations 102-104 are in communication with each other through abackhaul 105 which may include wired and/or wireless portions.

For the example of FIG. 1 , a UE device 106 is receiving wirelessservice from a first base station (first gNB) 102 where conditionschange sufficiently to warrant a handover to a second base station 104.For the example, the UE device 106 transmits a measurement report to thefirst base station when the uplink channel for transmission is clear.Therefore, the UE device 106 performs a listen-before-talk (LBT)procedure and determines when the channel is clear. The UE device 106may measure an energy level present in the channel and compare themeasured level to a threshold over a specified time period to determineif the channel is occupied. The channel may be occupied by other UEdevices operating in the system 100 or by other devices 114 using theunlicensed band that are not communicating within the system 100.

FIG. 2 is a block diagram of an example of a base station 200 suitablefor use as each of the base stations 102-104. The base station 200includes a controller 204, transmitter 206, and receiver 208, as well asother electronics, hardware, and code. The base station 100 is anyfixed, mobile, or portable equipment that performs the functionsdescribed herein. The various functions and operations of the blocksdescribed with reference to the base stations 102-104, 200 may beimplemented in any number of devices, circuits, or elements. Two or moreof the functional blocks may be integrated in a single device, and thefunctions described as performed in any single device may be implementedover several devices. The base station 200 may be a fixed device orapparatus that is installed at a particular location at the time ofsystem deployment. Examples of such equipment include fixed basestations or fixed transceiver stations. In some situations, the basestation 200 may be mobile equipment that is temporarily installed at aparticular location. Some examples of such equipment include mobiletransceiver stations that may include power generating equipment such aselectric generators, solar panels, and/or batteries. Larger and heavierversions of such equipment may be transported by trailer. In still othersituations, the base station 200 may be a portable device that is notfixed to any particular location. Accordingly, the base station 200 maybe a portable user device such as a communication device in somecircumstances. Although the base station may be referred to by differentterms, the base station is typically referred to as a gNodeB or gNB whenoperating in accordance with one or more communication specifications ofthe 3GPP directed to (NR-U SA) operation.

The controller 204 includes any combination of hardware, software,and/or firmware for executing the functions described herein as well asfacilitating the overall functionality of the base station 200. Anexample of a suitable controller 204 includes code running on amicroprocessor or processor arrangement connected to memory. Thetransmitter 206 includes electronics configured to transmit wirelesssignals. In some situations, the transmitter 206 may include multipletransmitters. The receiver 208 includes electronics configured toreceive wireless signals. In some situations, the receiver 208 mayinclude multiple receivers. The receiver 208 and transmitter 206 receiveand transmit signals, respectively, through an antenna 210. The antenna210 may include separate transmit and receive antennas. In somecircumstances, the antenna 210 may include multiple transmit and receiveantennas.

The transmitter 206 and receiver 208 in the example of FIG. 2 performradio frequency (RF) processing including modulation and demodulation.The receiver 208, therefore, may include components such as low noiseamplifiers (LNAs) and filters. The transmitter 206 may include filtersand amplifiers. Other components may include isolators, matchingcircuits, and other RF components. These components in combination orcooperation with other components perform the base station functions.The required components may depend on the particular functionalityrequired by the base station.

The transmitter 206 includes a modulator (not shown), and the receiver208 includes a demodulator (not shown). The modulator modulates thesignals to be transmitted as part of the downlink signals and can applyany one of a plurality of modulation orders. The demodulator demodulatesany uplink signals received at the base station 200 in accordance withone of a plurality of modulation orders.

For the example, the base station 200 includes a communication interface212 for transmitting and receiving messages with other base stations.The communication interface 212 may be connected to a backhaul ornetwork enabling communication with other base stations. In somesituations, the link 116 between the base stations 112, 114 may includeat least some wireless portions. The communication interface 212,therefore, may include wireless communication functionality and mayutilize some of the components of the transmitter 206 and/or receiver208.

The base station 200 has the capability to observe one or more downlinkchannels to determine if the channel is currently occupied. In otherwords, the base station 200 can determine whether other devices aretransmitting within the channel. For the examples herein, the receiver208 detects energy within the channel and the controller makes adetermination of whether the measured energy indicates the channel is inuse. For example, the measured energy can be compared to a threshold.Other techniques can be used in some circumstances.

FIG. 3 is a block diagram of an example of a UE communication device 300suitable for use as each of the communication device 106-109 of FIG. 1 .In some examples, the communication device 300 is any wirelesscommunication device such as a mobile phone, a transceiver modem, apersonal digital assistant (PDA), a tablet, or a smartphone. In otherexamples, the communication device 300 is a machine type communication(MTC) communication device or Internet-of-Things (IOT) device. Thecommunication device 300, (106-109), therefore is any fixed, mobile, orportable equipment that performs the functions described herein. Thevarious functions and operations of the blocks described with referenceto communication device 300 may be implemented in any number of devices,circuits, or elements. Two or more of the functional blocks may beintegrated in a single device, and the functions described as performedin any single device may be implemented over several devices.

The communication device 300 includes at least a controller 302, atransmitter 304 and a receiver 306. The controller 302 includes anycombination of hardware, software, and/or firmware for executing thefunctions described herein as well as facilitating the overallfunctionality of a communication device. An example of a suitablecontroller 302 includes code running on a microprocessor or processorarrangement connected to memory. The transmitter 304 includeselectronics configured to transmit wireless signals. In some situations,the transmitter 304 may include multiple transmitters. The receiver 306includes electronics configured to receive wireless signals. In somesituations, the receiver 306 may include multiple receivers. Thereceiver 304 and transmitter 306 receive and transmit signals,respectively, through antenna 308. The antenna 308 may include separatetransmit and receive antennas. In some circumstances, the antenna 308may include multiple transmit and receive antennas.

The transmitter 304 and receiver 306 in the example of FIG. 3 performradio frequency (RF) processing including modulation and demodulation.The receiver 304, therefore, may include components such as low noiseamplifiers (LNAs) and filters. The transmitter 306 may include filtersand amplifiers. Other components may include isolators, matchingcircuits, and other RF components. These components in combination orcooperation with other components perform the communication devicefunctions. The required components may depend on the particularfunctionality required by the communication device.

The transmitter 306 includes a modulator (not shown), and the receiver304 includes a demodulator (not shown). The modulator can apply any oneof a plurality of modulation orders to modulate the signals to betransmitted as part of the uplink signals. The demodulator demodulatesthe downlink signals in accordance with one of a plurality of modulationorders.

The UE communication device 300 has the capability to observe one ormore uplink channels to determine if the channel is currently occupied.In other words, the UE communication device 300 can determine whetherother devices are transmitting within the channel. For the examplesherein, the receiver 304 detects energy within the channel and thecontroller makes a determination of whether the measured energyindicates the channel is in use. For example, the measured energy can becompared to a threshold. Other techniques can be used in somecircumstances.

FIG. 4 is a timing diagram of communication for an example where aconditional handover is authorized by the first base station 102 in acommunication system utilizing unlicensed frequency bands fortransmission. The example of FIG. 4 begins with the UE communicationdevice 106 sending a measurement report at transmission 402. Aftermeasuring the appropriate channel parameters and determining theunlicensed uplink channel is clear, the UE device 402 sends themeasurement report to the first base station 102 which is the servingbase station in the example. In some circumstances, the UE device 106determines if measurement report trigger criteria have been met beforetransmitting the measurement report is transmitted. Examples of suchevaluations and criteria are discussed below.

At event 404, the first base station 102 performs an early handoverdecision. To support an early handover decision, the first base station102 must prepare all potential target base stations. This preparationincludes sending the target base stations the UE's context as well asthe Admission Control procedure 408. This is necessary since the UEdevice 106 is not ready to perform a handover at that time. Therefore,any one of the prepared target base stations can potentially become thebase station to which the UE device 106 is handed over.

At transmission 406, the first base station 102 sends a handover requestto the second base station 104 which is the target base station. For theexample, communication between the base stations is through the backhaul105 which does not utilize the unlicensed channels.

At event 408, the second bases station performs admission control. Theadmission control procedure admits or rejects the establishmentrequests, including handovers for new radio bearers. In order to dothis, admission control takes into account the overall resourcesituation in the network, the QoS requirements, the priority levels andthe provided QoS of in-progress sessions and the QoS requirement of thenew radio bearer request.

At transmission 410, the second base station sends a handoveracknowledgment to the first base station. The target base station sendsthe handover acknowledgement after the required resources for allaccepted radio bearers are allocated. If no resources are available onthe target side, the target eNB responds with the handover NACK messageor the handover failure message instead.

At event 412, the first base station performs an LBT procedure byobserving the unlicensed downlink channel to determine if it isoccupied. When the channel is unoccupied, the first base stationtransmits a conditional handover command to the UE device attransmission 414. The conditional handover command indicates that ahandover is authorized for a limited time period. For the example, theconditional handover command includes a time period indicator thatindicates the length of the time period. In some situations, the timeperiod indicator may include information that directly indicates thetime period. In other situations, the time period indicator may includea reference value that corresponds to a time period and that can beassociated with the time period by the UE device. In still othersituations, there may be a single preconfigured time period that is usedwhen the conditional handover command is received by the UE device. Thetime period may be configured to the UE device via dedicated signalingor system information. The time period is selected such that it is longenough to allow the UE device sufficient time to complete the handoverwhen the channel to the second base station (target gNB) is notoccupied. If the timer (time period) expires before a handover iscompleted, the serving gNB assumes that the conditional handover was notsuccessful. In such a situation, the UE device may be configured withanother handover scheme if the UE device is still reachable by theserving gNB. Otherwise, the UE device may declare radio link failure(RLF), transition to IDLE mode, or go to the RRC_INACTIVE state,depending on how the UE device is configured.

At event 416, the UE device makes a decision to perform a handover. Thedecision may be triggered by the original conditions when themeasurement report was sent or may be, at least partially, based on achange in conditions. Therefore, the threshold for triggering themeasurement report for a conditional handover may be different from aconventional handover threshold. The early decision by the first basestation (serving gNB) to send the conditional handover command providesthe UE device with additional flexibility on when to access the targetcell as compared to conventional systems. With conventional techniques,it may not be possible to send the measurement report when needed due tothe required LBT procedures. Also, with traditional techniques, theremay be delays in sending a handover command due to LBT procedures. Withthe conditional handover, the trigger to handover to the second basestation (target gNB) is still based on the configured measurementthreshold. Embedded within the conditional handover is anothermeasurement threshold that may be different from the measurementthreshold configured to the UE device 106 for the purposes ofmeasurement report. The measurement threshold within the conditionalhandover tells the UE device at what radio signal level the UE deviceshould trigger the handover to the target base station. Since there is atime period for performing the handover, however, the UE device is notrequired to send the measurement report and receive a handover commandat the time when the measured signal strength requires an immediatehandover. The signaling has been performed before the time the handoveris required. This will also reduce the possibility of handover failuresas this procedure reduces the possibility that the handover command isnot received at the UE device due to either weak connection to the firstbase station (source gNB) or LBT failure of the channel operated by thefirst base station. Therefore, this allows flexibility in terms of bothLBT and the configuration of the measurement threshold. Even if themeasurement threshold is satisfied, LBT procedure requirements mightprohibit the UE from accessing the target cell with the highest signalstrength. In this case the UE may handover to another target basestation that is unoccupied even if the signal strength is weaker.

At event 418, the UE device performs an LBT check. The UE deviceobserves the unlicensed uplink channel that will be used fortransmission and determines if the channel is unoccupied. The UE devicemay determine, for example, whether the level of measured energy in thechannel is above a threshold.

When the channel is determined to be unoccupied, the UE device performssynchronization to the target gNB and accesses the target cell via therandom access transmission 420 as part of the random access procedurewhich will also include UL resource allocation, timing alignment fromthe second base station. For the example, the UE device uses the targetgNB's specific keys and selected security algorithms. In case thechannel operated by the target gNB is occupied, the UE device may informthe source gNB of the channel occupancy measurement so that the sourcegNB may configure the UE device with an updated conditional handovercommand or to cancel the existing conditional handover command.

At event 422, the UE device performs an LBT check. The UE deviceobserves the unlicensed uplink channel that will be used fortransmission and determines if the channel is unoccupied. The UE devicemay determine, for example, whether the level of measured energy in thechannel is above a threshold.

When the channel is determined to be unoccupied, the UE device transmitsthe handover complete message (RRC Connection Reconfiguration Completemessage) to the second base station (target) at transmission 420 toconfirm the handover along with the UE device's buffer status report orUL data to the second base station. This indicates that the handoverprocedure is completed and the second base station is now the sourcebase station to the UE device.

FIG. 5 is a flow chart of an example of a method of managing handoverswith conditional handover commands. The method is performed by a UEdevice operating in a NR-U SA system such as the system 100 describedabove. Accordingly, the method may be performed by the UE device 106.

At step 502, a measurement for generating a measurement report is made.In accordance with instructions from the serving base station (firstbase station), the UE device determines parameters of received signalsfrom the serving base station and at least one target base station. Forevent-triggered measurement, measurement reports are only sent if themeasured value(s) are above the configured thresholds. Therefore, themeasurement report trigger conditions must be met before UE device inorder for transmission of the measurement report. Examples of suchprocedures and criteria are discussed below with reference to FIG. 8 .

At step 504, the unlicensed uplink channel is observed. The UE deviceperforms an LBT process to determine if the channel is occupied.

At step 506, it is determined where the uplink channel is occupied.Based on the results measured at step 504, the UE device determineswhether the channel is occupied. For example, one or more measuredenergy parameters of the channel may be compared to a threshold. If thechannel is occupied, the UE continues to observe the channel and returnsto step 504. Otherwise, the method continues to step 508. If the uplinkchannel continues to be occupied, the measurement performed in step 502may no longer be valid since the UE device's current measurement may nolonger be above the configured thresholds. The serving base station mayconfigure the UE device with a condition when the previous event triggermay still be used to send the measurement report (even if the currentmeasurement is no longer above the configured threshold). This conditionmay be a new timer that defines the validity time of the initialmeasurement report trigger. Alternatively, the condition may be definedas range value “alpha” dB below the measurement threshold. If thecurrent measurement value is more than alpha dB below the configuredthreshold, the UE device does not attempt to send the measurement reporteven if the channel is unoccupied. In this case, the procedure will berestarted at step 502.

At step 508, the measurement report is transmitted to the first basestation (serving gNB). The measurement process and the transmission ofthe measurement report may be triggered by conditions established by thenetwork.

At step 510, a conditional handover command is received. As discussedabove, the conditional handover command indicates that a handover isauthorized for a limited time period. For the example, the conditionalhandover command includes a time period indicator that indicates thelength of the time period. The UE device starts a conditional handovertimer which has a length equal to the conditional handover time periodindicated in the conditional handover command.

At step 512, it is determined whether a handover trigger has been met.The UE device observes channel conditions and determines whether thehandover procedure should be performed. The UE device continues tomonitor conditions if the trigger has not been met. Otherwise, themethod proceeds to step 514.

At step 514, the unlicensed uplink channel to the second base station isobserved.

At step 516, it is determined whether the uplink channel to the targetbase station (second base station) is occupied. Based on the resultsmeasured at step 514, the UE device determines whether the channel isoccupied. For example, one or more measured energy parameters of thechannel may be compared to a threshold. If the channel is occupied, theUE device continues to observe the channel and returns to step 514.Otherwise, the method continues to step 518.

At step 518, it is determined whether the conditional handover timer hasexpired. If the timer has expired, the method proceeds to step 520. Ifthe timer has not expired, the method proceeds to step 522.

At step 520, alternate handover management procedures are performed. Thealternate handover procedure may include any combination of direct andcontingent steps to execute in the event the conditional handover timerexpires. In some situations, the UE device may be configured withanother handover scheme if the UE device is still reachable by theserving gNB. In another examples, the UE device may declare radio linkfailure (RLF), transition to IDLE mode, or go to the RRC_INACTIVE state.The particular procedure invoked and sequence of steps depend on theparticular configuration for the UE device.

At step 522, an uplink synchronization and random access process isperformed. The process begins with the synchronization to the secondbase station (target) via the random access procedure. The process willfurther include UL resource allocation and timing alignment from thetarget base station.

At step 524, the unlicensed uplink channel to the second base station isobserved.

At step 526, it is determined whether the uplink channel to the targetbase station (second base station) is occupied. Based on the resultsmeasured at step 524, the UE device determines whether the channel isoccupied. For example, one or more measured energy parameters of thechannel may be compared to a threshold. If the channel is occupied, theUE device continues to observe the channel and returns to step 524.Otherwise, the method continues to step 528.

At step 528, a handover complete message is transmitted to the secondbase station. This indicates that the handover procedure is completedand the second base station is now the source base station to the UEdevice.

FIG. 6 is a timing diagram of communication for an example where ahandover command is provided with an inactivity indicator by the firstbase station 102 in a communication system utilizing unlicensedfrequency bands for transmission. The example of FIG. 6 begins with theUE communication device 106 sending a measurement report at transmission602. After measuring the appropriate channel parameters and determiningthe unlicensed uplink channel is clear, the UE device 106 sends themeasurement report to the first base station 102 which is the servingbase station in the example.

At event 604, the first base station 102 performs a handover decision.The source base station makes handover decision based on the measurementreport and radio resource management (RRM) information.

At transmission 606, the first base station 102 sends a handover requestto the second base station 104 which is the target base station. For theexample, communication between the base stations is through the backhaul105 which does not utilize the unlicensed channels.

At event 608, the second bases station performs admission control. Theadmission control procedure admits or rejects the establishmentrequests, including handovers for new radio bearers. In order to dothis, admission control takes into account the overall resourcesituation in the network, the QoS requirements, the priority levels andthe provided QoS of in-progress sessions and the QoS requirement of thenew radio bearer request.

At transmission 610, the second base station sends a handoveracknowledgment to the first base station. The target base station sendsthe handover acknowledgement after the required resources for allaccepted radio bearers are allocated. If no resources are available onthe target side, the target eNB responds with the handover NACK messageor the handover failure message instead.

At event 612, the first base station performs an LBT procedure byobserving the unlicensed downlink channel to determine if it isoccupied. When the channel is unoccupied, the first base stationtransmits a handover command with an inactivity indicator to the UEdevice at transmission 614. The handover command indicates that if thehandover fails due to the channel being occupied, the UE device shouldtransition to the inactive state (RRC_INACTIVE) and then resume activestatus. When re-establishing active status from the inactive state, theUE device should provide the I-RNTI allocated by the last serving basestation (the first base station 102) which may be used by the targetbase station (second base station 104) to retrieve the UE context fromthe last serving base station. As long as the UE device reselects a cellthat is within the RNA, the target base station should be able toretrieve the UE context.

At event 616, the UE device attempts a handover that fails.

At event 618, the UE device transitions to the inactive state(RRC_INACTIVE). After entering the inactive state, the UE devicetransitions back to the active state.

At event 620, the UE device performs an LBT check. The UE deviceobserves the unlicensed uplink channel that will be used fortransmission and determines if the channel is unoccupied. The UE devicemay determine, for example, whether the level of measured energy in thechannel is above a threshold.

When the channel is determined to be unoccupied, the UE device transmitsthe RRC connection resume request message to the second base station(target) at transmission 622. An RRC connection resume procedure is usedat transition from RRC_INACTIVE to RRC_CONNECTED where previously storedinformation in the UE as well as in the base station is utilized toresume the RRC connection. In the message to resume, the UE provides aResume ID (or I-RNTI) to be used by the target base station to accessthe stored information required to resume the RRC connection.

At transmission 624, the second base station 104 (target gNB) therequests UE context from the first base station 102 (source gNB). IntergNB connection resumption is handled using context fetching, whereby thetarget gNB retrieves the UE context from the source gNB over the Xninterface by sending the Retrieve UE Context Request message. The newgNB provides the Resume ID or I-RNTI which is used by the source gNB toidentify the UE context.

At transmission 626, the first base station 102 (source gNB) sends theUE context to the second base station 104 (target gNB) via the RetrieveUE Context Response message.

At event 628, the second base station performs an LBT check. The secondbase station device observes the unlicensed downlink channel that willbe used for transmission and determines if the channel is unoccupied.The second base station may determine, for example, whether the level ofmeasured energy in the channel is above a threshold. The second basestation also may determine whether it has sufficient resource to handlethe connection resume request based on the type of service and theamount of resource needed to handle the connection.

When the channel is determined to be unoccupied, the second base stationtransmits the RRC connection resume message to the UE device attransmission 630. In the case the UE context cannot be retrieved fromthe first base station (source gNB) or if the second base station cannotsupport the connection due to resource limitation, the second basestation may send an RRC Reject message to deny the UE device's resumerequest, in which case the UE device will perform the fallback procedureby transitioning to RRC_IDLE. While in RRC_IDLE the UE device mayinitiate a new connection, but the previously established UE contextinformation will be lost.

FIG. 7 is a flow chart of an example of a method of managing handoverswith handover commands having inactivity indicators. The method isperformed by a UE device operating in a NR-U SA system such as thesystem 100 described above. Accordingly, the method may be performed bythe UE device 106.

At step 702, a measurement for generating a measurement report is made.In accordance with instructions from the serving base station (firstbase station), the UE device determines parameters of received signalsfrom the serving base station and at least one target base station. Forevent-triggered measurement, measurement reports are only sent if themeasured value(s) are above the configured thresholds.

At step 704, the unlicensed uplink channel is observed. The UE deviceperforms an LBT process to determine if the channel is occupied.

At step 706, it is determined whether the uplink channel is occupied.Based on the results measured at step 704, the UE device determineswhether the channel is occupied. For example, one or more measuredenergy parameters of the channel may be compared to a threshold. If thechannel is occupied, the UE continues to observe the channel and returnsto step 704. Otherwise, the method continues to step 708. If the uplinkchannel continues to be occupied, the measurement performed in step 702may no longer be valid since the UE device's current measurement may nolonger be above the configured thresholds. The serving base station mayconfigure the UE device with a condition when the previous event triggermay still be used to send the measurement report (even if the currentmeasurement is no longer above the configured threshold). This conditionmay be a new timer that defines the validity time of the initialmeasurement report trigger. Alternatively, the condition may be definedas range value “alpha” dB below the measurement threshold. If thecurrent measurement value is alpha dB below the configured threshold,the UE will not attempt to send the measurement report even if thechannel is unoccupied. In this case, the procedure will be restarted atstep 702.

At step 708, the measurement report is transmitted to the first basestation (serving gNB). The measurement process and the transmission ofthe measurement report may be triggered by conditions established by thenetwork.

At step 710, a handover command with an inactivity indicator isreceived. As discussed above, the handover command with inactivityindicator indicates that the UE device should perform an inactivity andresume procedure if the handover fails due to unavailable channels.

At step 712, it is determined whether the handover failed due to theuplink channel being occupied. The UE device continues with standardcommunication and call management at step 714 if the handover issuccessful or if handover failure is due to a reason other than anoccupied channel. Otherwise, the method proceeds to step 716. In somesituations, the standard procedure at step 714 may include theinitialization of the RRC connection re-establishment procedure towardsa target base station that is considered a suitable cell.

At step 716, the UE device transitions to the inactive state. For theexample, the UE device transitions to an RRC_INACTIVE state defined byone or more revisions of a 3GPP communication standard.

At step 718, the unlicensed uplink channel to the second base station isobserved.

At step 720, it is determined whether the uplink channel to the targetbase station (second base station) is occupied. Based on the resultsmeasured at step 718, the UE device determines whether the channel isoccupied. For example, one or more measured energy parameters of thechannel may be compared to a threshold. If the channel is occupied, theUE device continues to observe the channel and returns to step 718.Otherwise, the method continues to step 722.

At step 722, the UE device transmits a connection resume request to thesecond base station (target gNB).

At step 724, the UE device receives a connection resume message from thesecond base station (target gNB). This indicates that the handoverprocedure is completed and the second base station is now the sourcebase station to the UE device. The UE device continues withcommunication with the second base station as the new serving basestation (source gNB).

FIG. 8 is a flow chart of an example of a method of managing measurementreport transmission. The method is performed by a UE device operating ina NR-U SA system such as the system 100 described above. Accordingly,the method may be performed by the UE device 106. The method is anexample of performing steps 502 through 508 of the method of FIG. 5 andis an example of performing steps 702 through 708 of the method of FIG.7 .

At step 802, a measurement report trigger information is received. Thesource base station (source gNB) generates and transmits the measurementreport trigger information which indicates to the UE device the criteriafor transmitting the measurement report of the source base station. Themeasurement report trigger information may define thresholds forreceived signals. Examples of signal thresholds include a power level ofreceived signal from non-serving base stations such as Reference SignalReceived Power (RSRP) level, a quality level of received signal fromnon-serving base stations such as Reference Signal Received Quality(RSRQ) level, a power level of received signal from the serving basestation such as RSRP level, a quality level of received signal from theserving base station RSRQ level, and any combination thereof. Themeasurement report trigger information may also include a thresholdoffset value for one or more of the threshold values where the thresholdoffset value may be different for each threshold. In addition, themeasurement report trigger information may also include a validity timerindicating a time period from the transmission for which the measurementreport trigger information will be valid.

At step 804, the channel measurement is performed. Signals are receivedfrom non-serving base station and/or serving the serving base station.Quality and/or power measurements are performed on the received signals.

At step 806, it is determined is the trigger criteria for the channelmeasurement is performed. The signals received from non-serving basestation and/or serving the serving base station are compared to thethresholds. Other criteria, such as timers and offsets are alsoevaluated to determine if the criteria for transmitting the measurementreport has been met. If the criteria have been met, the method continuesat step 808. Otherwise, the method returns to step 804.

At step 808, the unlicensed uplink channel is observed. The UE deviceperforms an LBT process to determine if the channel is occupied.

At step 810, it is determined whether the uplink channel is occupied.Based on the results measured at step 808, the UE device determineswhether the channel is occupied. For example, one or more measuredenergy parameters of the channel may be compared to a threshold. If thechannel is occupied, the UE continues to observe the channel and returnsto step 808. Otherwise, the method continues to step 812.

At step 812, it is determined whether the measurement trigger criteriaare still being met. For example, the timer period since thetransmission of the measurement report trigger information may becompared to the validity timer to determine if the trigger criteria arestill valid. In addition, new criteria may have been received since themeasurement had been taken and the measurement must be evaluated againstthe new criteria. If the trigger criteria are met, the measurementreport is transmitted at step 814. Otherwise, the method returns to step804. New measurement report trigger information may be received duringthe process other than at step 802.

Clearly, other embodiments and modifications of this invention willoccur readily to those of ordinary skill in the art in view of theseteachings. The above description is illustrative and not restrictive.This invention is to be limited only by the following claims, whichinclude all such embodiments and modifications when viewed inconjunction with the above specification and accompanying drawings. Thescope of the invention should, therefore, be determined not withreference to the above description, but instead should be determinedwith reference to the appended claims along with their full scope ofequivalents.

The invention claimed is:
 1. A method performed at a communicationdevice, the method comprising: receiving, from a serving gNB,measurement report trigger information indicative of criteria forsending a measurement report, the criteria comprising: a measurementreport trigger threshold indicative of at least one of a minimumreceived signal strength threshold and a minimum received signal qualitythreshold; a measurement report trigger validity period indicative of atime period that the measurement report trigger threshold is valid, thetime period from receipt of the measurement report trigger information;and a threshold offset value indicative of a maximum offset below themeasurement report trigger threshold after expiration of the measurementreport trigger validity period; evaluating a channel occupancy level ofan unlicensed channel to the serving gNB; and transmitting, if thechannel occupancy level is below a channel occupancy threshold and onlyif the criteria are met, a measurement report over the unlicensedchannel to the serving gNB, the measurement report indicative of atleast one of a received signal strength and a received signal quality atthe communication device of at least one received signal transmittedfrom the target gNB; receiving, from the serving gNB, a handover commandwith an inactivity indicator, the handover command identifying ahandover procedure to a target gNB; in response to failing the handoverprocedure at least partially as a result of another unlicensed channelto the target gNB being occupied, transitioning to a New Radio (NR)Radio Resource Control (RRC) Inactive state; and executing an RRC Resumeprocedure to transition to the RRC Connected state to establishcommunication service with a new serving gNB located within a same RadioAccess Network (RAN)-based Notification Area (RNA) as the serving gNB.2. The method of claim 1, wherein the new serving gNB is the target gNB.3. The method of claim 1, further comprising: transmitting, over theanother unlicensed channel, an I-Radio Network Temporary Identifier(I-RNTI) to the new serving gNB.
 4. The method of claim 1, furthercomprising: in response to failing the handover procedure as a result ofa radio link failure (RLF), executing a NR RRC re-establishmentprocedure.
 5. The method of claim 1, further comprising: transmitting,to the serving gNB and in response to a determination that the occupancylevel is not below the channel occupancy threshold, occupancyinformation indicative of the occupancy level.
 6. A user equipmentcommunication device (UE device) comprising: a receiver configured toreceive, from a serving gNB, measurement report trigger informationindicative of criteria for sending a measurement report, the criteriacomprising: a measurement report trigger threshold indicative of atleast one of a minimum received signal strength threshold and a minimumreceived signal quality threshold; a measurement report trigger validityperiod indicative of a time period that the measurement report triggerthreshold is valid, the time period from receipt of the measurementreport trigger information; and a threshold offset value indicative of amaximum offset below the measurement report trigger threshold afterexpiration of the measurement report trigger validity period; acontroller configured to evaluate an occupancy level of an unlicensedchannel to the serving gNB, a transmitter configured to transmit, if theoccupancy level of the unlicensed channel to the serving gNB is below achannel occupancy threshold and only if the criteria are met, ameasurement report over the unlicensed channel to the serving gNB, themeasurement report indicative of at least one of a received signalstrength and a received signal quality at the UE device of at least onereceived signal transmitted from the target gNB, the receiver configuredto receive a handover command with an inactivity indicator, the handovercommand identifying a handover procedure to a target gNB, the controllerconfigured to: in response to failing the handover procedure at leastpartially as a result of another unlicensed channel to the target gNBbeing occupied, transition to a New Radio (NR) Radio Resource Control(RRC) Inactive state; and execute an RRC Resume procedure to transitionto the RRC Connected state to establish communication service with a newserving gNB located within a same Radio Access Network (RAN)-basedNotification Area (RNA) as the serving QNB.
 7. The UE device of claim 6,wherein the new serving gNB is the target gNB.
 8. The UE device of claim6, further comprising: a transmitter configured to transmit, over theanother unlicensed channel, an I-Radio Network Temporary Identifier(I-RNTI) to the new serving gNB.
 9. The UE device of claim 6, thecontroller further configured to execute a NR RRC re-establishmentprocedure in response to failing the handover procedure as a result of aradio link failure (RLF).
 10. The UE device of claim 6, the transmitterconfigured to transmit, to the serving gNB and in response to adetermination that the occupancy level is not below the channeloccupancy threshold, occupancy information indicative of the occupancylevel.